The present invention relates generally to molded case circuit breakers. More specifically, the present invention relates a bottom vented circuit breaker capable of top down assembly onto equipment.
Conventional circuit breakers include a breaker casing with one or more pairs of separable contacts disposed therein. Typically, each pole or phase of the breaker employs a single contact pair comprising one fixed and one movable contact. The fixed contact is electrically connected to a breaker connection lug extending within an access opening formed on the line side of the case. The movable contact is electrically connected to a breaker connection lug extending within an access opening formed on the load side of the case. The breaker connection lugs are electrically connected to equipment connection lugs extending within the access openings and forming part of the protected circuit. The access openings allow the assembled circuit breaker to be installed in a top-down manner, onto the equipment connection lugs, with the equipment connection lugs passing through the access openings as the breaker is lowered onto the equipment.
In all circuit breakers, the separation of the breaker contacts due to a short circuit causes an electrical arc to form between the separating contacts. The arc causes the formation of relatively high-pressure gases as well as ionization of air molecules within the circuit breaker. These high-pressure gases can cause damage to the breaker casing. The gases, therefore, must be vented from the circuit breaker enclosure. In addition, a phase-to-phase fault can occur if the arc gases from different phases are allowed to mix, and a phase-to-ground fault can occur if the gases contact the grounded enclosure. To avoid a phase-to-phase or phase-to-ground fault, gases vented from different phases must be kept separate from each other and away from the grounded enclosure until the ionization has dissipated.
In a conventional circuit breaker, when an overcurrent condition is present in the protected circuit, the movable contact moves upward under the force of an operating mechanism mechanically attached thereto. This action generates arc gases above the fixed contact and its associated breaker connection lug. These gases are propelled towards the top corner of the breaker. Thus, the breaker case is designed with an arc gas vent located near the top corner, to vent the arc gases to atmosphere. Typically, the arc gas vent is located on the line side of the breaker case, above the breaker connection lug. Because the arc gas vent is located near the top of the case, above the breaker connection lug, there is no interference between the arc gas vent and the access opening, which is located beneath the breaker connection lug.
However, many modern molded case circuit breakers employ a contact arrangement where arc gases are frowned beneath the equipment and breaker connection lugs. A rotary contact arrangement is an example of such a contact arrangement. In rotary contact breakers, each pole or phase employs two sets of contacts: two fixed contacts that are fixed relative to the breaker case, and two movable contacts that rotate about a common axis generally perpendicular to the current path from the line side to the load side of the circuit breaker. When the movable and fixed contacts separate, the load-side movable contact moves downward, away from the load-side fixed contact. This action generates arc gases beneath the load-side lugs, and propels the gases towards the bottom corner of the breaker casing. Thus, it is desirable for the arc gas vent to be located at the bottom of the load side of the breaker casing. Unfortunately, such a location would interfere with the access openings and, therefore, the ability to install the breaker in a top-down manner.